1. Field of the Invention
The present invention relates to a handover between a networks having different radio access technologies, in particular in which a first network supports a packet switched (PS) connection, and a second network supports a packet switched connection and a circuit switched (CS) connection. An example for the first network may be LTE (Long Term Evolution, also referred to as High Speed OFDM Packet Access (HSOPA), (OFDM=Orthogonal Frequency Division Multiplexing).
2. Description of the Related Art
In LTE, voice will be supported via PS data using IMS as default service deliverer (VoIMS). As LTE coverage will be limited during the initial LTE rollout, there is a need to continue providing the voice call through another pre-LTE system such as 3 G (UMTS) or 2 G (GSM/CPRS). Operators handling 2 G or 3 G networks may want to keep all voice services in the CS domain. Thus there is one challenge to solve: how to provide voice call continuity for a VoIMS in LTE to a CS domain in 2 G or 3 G for terminals with one receiver. This procedure is not yet standardized and is called Single Radio Voice Call Continuity (SR VCC). Several alternatives for this procedure have been included in TR 23.882 section 7.19: “Key Issue—Service continuity between IMS over SAE/LTE access and CS domain”. One of the SR VCC alternatives is called Combinational VCC and proposes to provide voice call continuity via two steps:
1. PS handover from LTE to 3 G/2 G PS domain
2. Domain change in the same system, from 2 G/3 G PS to 2 G/3 G CS domain.
In this step, voice is being handled in 2 G/3 G PS temporarily (VoIP). In the meantime, UE is requesting a voice call establishment over CS. Once CS voice bearer is established, UE can release the PS bearer. After this, UE handles voice over CS.
This involves at least two problems. The first problem concerns the time the UE is handling voice over PS in the target domain.
According to TS 23.206 section 4.5_“Domain transfer procedures” (between a non-3GPP and 3GPP CS domain), all UE make the decision about domain transfer. In the particular case for voice continuity between LTE to a 3 G/2 G CS domain, step 2 in section 1 describes the need for UE to spend in the PS domain. It is still an open issue in the 3GPP standardization when UE will trigger the PS to CS domain change. The time UE spend in the target PS domain should be kept as short as possible due to quality reasons e.g., quality of voice over 2 G/3 G PS domain may not be as good as in CS domain.
Therefore, there is a need to speed up the domain change so that UE stays in PS domain for the shortest time.
A second problem is that an impact on legacy radio and core network due to SR VCC may be caused.
If UE does not trigger the transfer domain, then either core network or the radio access should take care of the triggering which may lead to new procedures in legacy RN/CN networks. That is, such an impact on legacy radio and core network due to SR VCC should be avoided.
Therefore, UE triggering of the domain change is needed
The problems have not been detected so far because the basic SR VCC procedures are not specified. The combinational VCC approach appears in SA2 as the most attractive solution. If selected, then procedures to speed up the domain transfer from PS to CS will be required. The second problem has been included for voice call continuity between a non-3GPP system and a 3GPP CS (TS 23.206). The first problem has not been addressed at all heretofore.